Accelerated formation and increased performance in chemically pre-formed (CPF) iron negative electrodes

ABSTRACT

Provided is a process for preparing an electrode comprising an iron active material. The process comprises first fabricating an electrode comprising an iron active material, and then treating the surface of the electrode with an oxidant to thereby create an oxidized surface. The resulting iron electrode is preconditioned prior to any charge-discharge cycle to have the assessable surface of the iron active material in the same oxidation state as in discharged iron negative electrodes active material.

RELATED APPLICATION

The present application is a divisional of U.S. non-provisionalapplication Ser. No. 14/478,843 filed on Sep. 5, 2014, which claimspriority to provisional application U.S. No. 61/874,177 filed on Sep. 5,2013, which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of an electrode comprising an iron active material.

Related Art

Rechargeable batteries often require several charge-discharge cyclesprior to achieving optimum performance. During these early cycles,critical surface films are formed on the electrode surfaces that affectthe performance of the cell during later cycling. These early cycles arecommonly termed formation cycles in the battery industry. In the case ofnickel-iron batteries (Ni—Fe), 30 to 60 formation cycles are typicallyneeded to achieve the full capacity of the cell. Formation cyclingsometimes requires cycling at varied temperature regimes whichcomplicates the process. This formation process is expensive, timeconsuming, consumes electrolyte which needs replacing, and generates asignificant amount of gas. Therefore, reducing the number of formationcycles and simplifying the formation process is a worthy goal.

Manohar et. al. in “Understanding the factors affecting the formation ofCarbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries”, J.Electrochem. Soc., 159, 12, (2012) A 2148-2155, reported that one reasonfor the long formation time could also be the poor wettability of theiron electrode and the inaccessibility of the pores of the iron by theelectrolyte. As the pores became more accessible the charge anddischarge process produced a progressively rougher surface resulting inan increase in electrochemically active surface area and dischargecapacity. Triton X-100, a surfactant, reduced the number of cyclesrequired to achieve higher capacity presumably because it improvedaccess of the electrolyte to the pores.

U.S. Pat. No. 3,507,696 teaches that a mixture of FeO and Fe₂O₃ powdersfused with sulfur at 120° C. yields an active material that may be usedin an aqueous slurry to impregnate sintered nickel fiber plaques thatcan used as a negative electrode in a Ni—Fe battery. Several formationcycles are needed to achieve high capacity.

It would be of benefit to the industry to have an iron electrode whichis conditioned prior to any charge-discharge cycle so as to minimize theneed for formation cycles.

SUMMARY OF THE INVENTION

Provided is a process for preparing an electrode comprising an ironactive material, which comprises:

-   -   i) fabricating an electrode comprising an iron active material,        and    -   ii) treating the surface of the electrode with an oxidant to        thereby create an oxidized surface.        In one embodiment, the oxidant comprises water, hydrogen        peroxide, ozone, chlorine, nitric acid, hypochlorite, nitrous        oxide, bromine, iodine, permanganate compounds, or sodium        perborate.

In another embodiment, provided is an electrode which comprises an ironactive material, which electrode has been preconditioned prior to anycharge-discharge cycle to have the accessible surface of the ironmaterial in the same oxidation state as the discharged iron negativeelectrode active material. In one embodiment, the oxidation state of theconditioned iron active material is +2, +21/+3, +3 or +4.

Among other factors, the present invention provides a process andresulting iron electrode which addresses the mismatch in thestate-of-charge (SOC) of the anode and cathode that is present duringNi—Fe cell assembly. Use of the present iron electrode decreases thenumber of cycles, and time to achieve cell formation, electrolyteconsumption, hydrogen gas generated, and the amount of water needed torefill the cell. In general, the use leads to improved iron utilizationin the cell.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is an illustration of the interparticle contact between activematerial particles and the space between particles or pores that can befilled with oxidizing solution to precondition the surface of theelectrode particles.

FIG. 2 shows the capacity of Ni—Fe cells with and without preconditionediron electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provided by the present invention is a chemically preconditioned ironelectrode and a method for its preparation. The present inventionchemically treats an iron metal electrode after the electrode isassembled to provide a preconditioned iron electrode. It is expectedthat the process of the present invention is amenable to a continuousprocess, and is therefore simpler and of lower cost than existentprocesses.

The preconditioned electrode may be prepared from a standard ironelectrode used in Ni—Fe cells. These iron electrodes can be comprised ofiron particles or mixtures thereof with sulfur, nickel, or other metalpowders, bonded to a substrate. In one embodiment, a conductive additivefor the iron electrode comprises nickel, carbon black or copper. In oneembodiment, an additive of the iron electrode comprises sulfur. Inanother embodiment, the coating of active material of the iron electrodecomprises a binder for the iron or iron active material, and additives.The binder is generally a polymer such as PVA, or a rubber. The use of aPVA binder has been found to be quite beneficial and advantageous.

In one embodiment, the iron electrode comprises about 50-90 wt % ironpowder, and in another embodiment from about 75-85 wt % iron powder;from about 5-30 wt % nickel powder, and in another embodiment from about12-20 wt % nickel powder; from 0.5-5.0 wt % binder, and in anotherembodiment from about 2.0-5.0 wt % binder; and, from 0.25-2.0 wt %sulfur, and in another embodiment, from about 0.25-1.0 wt % sulfur. Inone embodiment, the iron electrode comprises about 80 wt % iron powder,about 16 wt % nickel powder, about 3.5 wt % binder and about 0.5 wt %sulfur powder.

In one embodiment, the iron electrode can comprise additionalconventional additives, such as pore formers. In general, the porosityof the iron electrode is in the range of from 15-50%, and in oneembodiment from 35-45%.

The substrate used in the electrode can be comprised of a conductivematerial such as carbon or metal. The substrate for the iron electrodeis generally a single layer of a conductive substrate coated on at leastone side with a coating comprising the iron active material. Both sidesof the substrate can be coated. In one embodiment, the coating on atleast one side comprises iron and additives comprised of sulfur,antimony, selenium, tellurium, nickel, bismuth, tin, or a mixturethereof. The substrate is generally a metal foil, metal sheet, metalfoam, metal mesh, woven metal or expanded metal. In one embodiment, thesubstrate for the iron electrode is comprised of a nickel plated steel.It is generally of porous construction such as that provided by a mesh,or grid of fibrous strands, or a perforated metal sheet. The ironelectrode can also be sintered.

The iron electrodes of the present invention are chemicallypreconditioned with oxidants that are able to oxidize the iron surface.These materials include but are not limited to: water, hydrogenperoxide, ozone, chlorine, nitric acid, hypochlorite, nitrous oxide,bromine, iodine, permanganate compounds, and sodium perborate. Oxidizingmaterials that are non-toxic and volatile and yield reduction or thermaldecomposition products that are also non-toxic and volatile arepreferred. Preferred oxidants are water and hydrogen peroxide. Water andsolutions used to pretreat the iron electrodes may or may not contain asurfactant. An example of a surfactant that may be used includes but isnot limited to Triton X-100. The treatment of the electrodes may beaccomplished by coating, dipping, spraying, or otherwise applyingoxidants or solutions containing the oxidant to the electrode. Theelectrode may also be preconditioned by exposing the electrode to anoxidizing gas.

The length of time the electrodes are treated can vary, but is generallyuntil oxidation of the iron on the accessible surface of the electrodeis observed. The temperature at which the treatment is made is generallyambient, but it can be at higher temperatures, eg., in an oven. Afterthe treatment, the electrode is dried. It can be air dried or in anoven, for example.

In one embodiment, the treatment of the iron electrode is continueduntil the accessible surface of the iron material of the electrode is inthe same oxidation state as the electrode would in the discharged state.This is achieved by the oxidation treatment and can be determined usingconventional methods available.

While not wishing to be bound by theory, it is believed that nickel-ironbatteries may sometimes be assembled with the nickel cathode (positiveelectrode) in its discharged state and the iron anode (negativeelectrode) in its charged state. Thus, when the cell is assembled thereis a mismatch between the state-of-charge (SOC) between the anode andcathode which is corrected during the formation process. During theformation process, it is believed that the low capacity of the earlycycles is due to the limited amount of discharge products (ie. Fe(OH)₂,Fe(OH)₃, and Fe₃O₄ depending on depth of discharge) that are formeduntil the proper conductivity, texture, and porosity of the ironelectrode is achieved. Consequently, the negative electrode is in ahigher SOC than the positive electrode for most of the formationprocess.

During the charge of a Ni—Fe cell there are typically two processes thatoccur at the anode surface, which are shown in Equations 1 and 2 below.Equation 1 is the desired conversion of discharge product, Fe(OH)₂, toiron metal. Equation 2 is the reduction of water to hydroxide andhydrogen gas. The two processes have very similar electrochemicalpotentials and both are usually active during the charge process.Fe(OH)₂+2e ⁻→Fe+2OH⁻ Eº=−0.877 V  12H₂O+2e ⁻→H₂+2OH⁻ Eº=−0.828 V  2

However, when the negative electrode is at high SOC as in formation, thereaction in Equation 2 is more dominant since there is too littleFe(OH)₂ or other iron compounds with iron in its +2 or +3 oxidationstate to accept current from the cathode. The reaction in Equation 2consumes the water in the electrolyte which needs to be replaced andgenerates significant amount of gas that can become trapped between theelectrodes, further hindering desired electrochemical reactions at theelectrode surfaces. Gas generation can cause loss of adhesion of theactive material to the electrode further damaging the electrode.

It is believed that chemically pretreating the electrode with oxidantsin accordance with the present invention converts areas of the electrodethat are accessible by the alkaline electrolyte, including pores, toiron hydroxides and/or iron oxides that are capable of being reduced toiron metal when an electrochemical current is applied in a cell. Theproducts of the pretreating of the iron electrode may be the same as thedischarge products on the iron electrode, or may be different. Theseproducts may comprise independently or as a mixture: Fe(OH)₂, Fe(OH)₃,Fe₃O₄, Fe₂O₃, FeO, and other iron oxides. As a result, the mismatch inthe SOC of the anode and cathode that is present during Ni—Fe cellassembly is minimized, if not avoided all together. Use of the presentiron electrode thereby decreases the number of cycles and time toachieve cell formation, electrolyte consumption, hydrogen gas generated,and the amount of water needed to refill the cell.

FIG. 1 shows a diagram of an electrode that has been preconditioned. Theiron particle active material, 1, retains interparticle contact, 2, andelectrical contact between the active materials and the substrate, 3, ismaintained. The surface of the electrode and the pores, 4, are able tobe contacted by the oxidant for preconditioning. Areas where there isinterparticle contact are not oxidized. Because the oxidation productsare electrically insulating, it is an advantage of this invention thatthe areas where there is interparticle contact are not oxidized,maintaining a conductive network between particles.

The present example is provided to further illustrate the presentinvention. It is not meant to be limiting.

Example

An aqueous slurry consisting of 80% iron, 16% nickel, and 0.5% sulfurpowders with 3.5% polyvinyl alcohol binder were pasted onto a perforatednickel sheet which was then dried. This sheet was then chemicallypreconditioned by brushing with deionized water and allowed to dry inair for 16 hours at room temperature followed by drying in an oven at190° C. for 15 minutes. A 16% weight gain was measured and a slightorange-brown color was observed on the surface of the electrode. Twosample electrodes were cut from this sheet and tabs were TIG welded tothe top uncoated area of the electrode. Two sample cells wereconstructed using these negative electrodes by placing the negativeelectrode between two commercial Histar sintered positive nickelhydroxide electrodes. Both the positive and negative electrodes werepocketed into polypropylene separator. For comparison, two identicalcells were constructed from identical materials except that the negativeelectrodes were not chemically preconditioned. The test cells containingCPF iron negative electrodes and the control cells were subjected to anaccelerate life test at 55° C. with the following charge regime:

Cycle 1 (@Room Temp):

-   -   Charge: 1.0 A×1.5 hrs    -   Rest: 30 Min    -   Discharge: 0.1 A to 1.0 V    -   Rest: 30 Min        Cycle 2-100    -   (@55° C.): Charge: 1.0 A×1.5 hrs    -   Rest: 30 Min    -   Discharge: 0.1 A to 1.0 V    -   Rest: 30 Min

The cycling characteristics for cells prepared with pre-conditioned ironelectrodes is shown in FIG. 2. The cells with chemically pre-conditionednegative electrodes deliver a capacity of 140-160 mAh/g Fe after onlyfive cycles compared to a capacity of 120-135 mAh/g Fe after ten cyclesfor cells with negative electrodes that were not preconditioned.Furthermore, the overall capacity for cells with preconditionedelectrodes is between 17-19% higher for the life of the cell afterformation demonstrating a further advantage of this invention.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of variations, combination, and equivalents ofthe specific embodiment, method, and examples therein. The inventionshould therefore not be limited by the above described embodiment,method and examples, but by all embodiments and methods within the scopeand spirit of the inventions and the claims appended therein.

What is claimed is:
 1. A process for preparing an electrode comprisingan iron active material, which comprises: i) fabricating an electrodecomprising an iron active material, a polyvinyl alcohol binder andsulfur, and ii) treating the surface of the electrode with an oxidant tothereby create an oxidized surface prior to the electrode beingsubjected to a charge-discharge cycle.
 2. The process of claim 1,wherein the oxidation state of the iron active material in the oxidizedsurface is +2, +2/+3, +3 or +4.
 3. The process of claim 1, wherein abinder is added to the electrode during the fabricating of theelectrode.
 4. The process of claim 1, wherein the sulfur is added to theelectrode during the fabricating of the electrode.
 5. The process ofclaim 1, wherein a conductive additive is added to the electrode duringthe fabricating of the electrode.
 6. The process of claim 5, wherein theconductive additive comprises nickel, or copper or carbon black.
 7. Theprocess of claim 1, wherein the fabricating of the electrode comprisesmixing an iron active material, nickel powder, a binder and sulfur. 8.The process of claim 7, wherein the fabricating of the electrode resultsin an electrode comprising: 50-90 wt % iron powder 5-30 wt % nickelpowder 0.5-5.0 w % binder, and 0.25-20 w % sulfur.
 9. The process ofclaim 1, wherein the fabricating of the electrode comprises coating asingle layer of the iron active material on at least one side of aconductive substrate.
 10. The process of claim 9, wherein the substrateis comprised of nickel plated steel.
 11. The process of claim 1, whereinthe porosity of the electrode fabricated in step i) is in the range ofabout 15-50%.